12/23/2023 0 Comments Logicworks rc circuit![]() Another option is to use a logic IC circuit shown above, configured in a similar way but I chose the 555 timer because it's more on topic. The minimum ripple valley (VC1 - one diode drop - Vripple) should be above the voltage required by MOSFET gate to allow it to pass the required current for the load, otherwise it will get hot. The circuit attached gives calculations which can be used to calculate the minimum load current and the maximum duty cycle. The disadvantage is that there's a minimum load requirement and the duty cycle can't be 100% as the capacitor charges when the switching transistor is off. It's better to use a CMOS 7555 because the lower current requirement means the capacitor is discharged less. Another thing: did you know that the whole 555 can be connected with a MOSFET in a bootstrapping configuration so the whole circuit can be just connected in series with the load? As well as simplifying wiring, it has the advantage of being able to do high side drive with an N-channel device which will be cheaper and have a lower on resistance. In my opinion, there's no point in using an MCU for this, unless it's already doing something else, it's purely for educational purposes or your project needs to use push-buttons because there isn't room for a pot. The solution of course is to use an MCU with an ADC but, again, that increases the cost. Using a MCU would need the variable resistor to be replaced with a couple of push-buttons which isn't as ergonomic as a pot and the value isn't saved unless you use an MCU with data flash memory which costs more.MCUs are more sensitive to noise than simple logic gates or a 555.MCUs are limited to a maximum current of 25mA per I/O pin and even if several I/O are paralleled (assuming this can be done) the limit for a port is 75mA. A 555 has a high current output capable of sourcing or sinking up to 200mA which is enough to drive a tiny motor directly, switch a MOSFET very quickly or give a power BJT plenty of base drive.Yes, I know that some MCUs have built-in voltage regulators but they're more expensive and the output high voltage is the regulated voltage, not the supply voltage. A MCU is more picky about the power supply voltage: 2V to 5.5V at best, a typical 555 can operate from 4.5V to 18V (TTL), CMOS 555s operate over 3V to 18V, HC logic works from 2V to 6V and some LV logic can work below 1V.The cheapest MCU is still more expensive than a single 555 or logic IC, even when external components are accounted for.MCUs (especially the cheap ones) also have many disadvantages compared to a simple 555 or logic even if one ignores the learning curve or requiring a programmer: The comparator circuit is a little more complicated but it's probably not worth it for a fixed duty cycle. The same applies to all the circuits with the logic gates. The frequency is equal to the normal 555 timer formula listed on the datasheet but the value of the pot is halved or if it's two resistors, it's equal to the mean. The resistor above the pot should be very small compared to the pot (too high and the minimum duty cycle will be limited) but not too small otherwise the 555 will overheat, 1k is 1% of 100k which is fine. It works the other way round too, if the part of the pot connected to the diode pointing up is 90k and the part of the pot connected to the diode pointing down is 10k, the duty cycle will be about 10%. When the part of the pot connected to the diode pointing down is highest (say 90k) and the part connected to the diode pointing up is lowest (say 10k), the duty cycle will be high (about 90%) because it will take longer for the capacitor to charge than it does to discharge. ![]() When the pot is at 50% of its setting the duty cycle will be 50% because the charge and discharge times will be the same. The pot can be split into two halves, because the diodes ensure the current takes a different path when charring than it does discharging.
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